1. Field of the Invention
This invention relates to the production of superconducting ceramics. More particularly, this invention relates to an improved process for mixing the reactants used in forming the superconducting ceramic.
2. Description of the Related Art
Since the discovery of superconductivity in 1911, the phenomena of a material being able to conduct electricity with almost no resistance when the material is cooled to a temperature approaching absolute zero (0.degree. K.) has remained an interesting scientific curiosity having few applications which would justify the expense of maintaining the necessary liquid helium cooled system.
Recently, however, superconducting ceramic materials have been produced which exhibit this phenomena at much higher temperatures, e.g., in some cases even higher than the boiling point of liquid nitrogen which boils at about 77.degree. K. The ability to produce superconductivity, for example, in a material cooled by liquid nitrogen completely changes the economics which have previously restricted the applications to which superconductivity could be applied.
These new ceramic materials are sometimes referred to as triple-layer perovskite compounds because of the crystallography of the resulting structure; or 1-2-3 compounds because of the atomic ratios of 1 atom of a rare-earth (Lanthanum series) element such as lanthanum or yttrium, 2 atoms of an alkaline earth metal such as barium or strontium, and 3 atoms of copper. The superconducting ceramic also contains from 6.5+ to 7- atoms of oxygen which is usually referred to as O.sub.(6.5+x) where x is greater than 0 and less than 0.5, resulting in a chemical formula such as, for example, YBa.sub.2 Cu.sub.3 O.sub.(6.5+x).
The prevalent method used to produce this type of superconducting ceramic is to mechanically mix powders of the oxides or carbonates of the respective rare earth, alkaline earth metal, and copper elements in the 1-2-3 structure of the superconductor, calcine the mixture to remove water or other volatiles, and then fire the powder mixture in an oxygen atmosphere at a temperature sufficiently high to produce the desired superconducting phase.
However, the inadequacies of the mixing process is evidenced by the variations in compositions of the resulting fired ceramic material, and consequently, variations in properties. The mixing process, usually in a ball mill, takes many hours, and sometimes introduces impurities from the balls or the ball milling vessel.
It would, therefore, be desirable to provide a more satisfactory method of mixing the reactants used in forming the superconducting ceramic in a manner which would produce more uniform results in the ceramic material formed from the reactant mixture.
It is known to co-precipitate more than one compound from a solution using a common anion such as, for example, an oxalate. However, if such a mixture of co-precipitated oxalate compounds was subsequently fired, a residue such as elemental carbon could be left which could then be a contaminant. Other co-precipitation processes may also involve the use of an anion which includes carbon or might use an anion containing another metal, e.g., a chromate.
The precipitation of rare earth carbonates by hydrolysis of rare earth trichloroacetates is discussed by Salutsky et al in "The Rare Earth Metals and their Compounds XII, Carbonates of Lanthanum, Neodymium, and Samarium", published in Volume 72 of the Journal of the American Chemical Society, at pages 3306-3307 in 1950. This type of formation of rare earth carbonates is also discussed by Head et al in an article entitled "The Preparation and Thermal Decompositions of Some Rare Earth Carbonates", published in the Proceedings of the Third conference on Rare Earth Research, at pp. 51-63 in 1963; and in another Head et al article "The Preparation and Thermal Decomposition of the Carbonates of Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, and Sc", published in the Proceedings of the Fourth conference on Rare Earth Research, at pp. 707-718 in 1964.